Since a water jet propulsion unit for a hull draws and injects water in the neighborhood of the water surface, it is blocked in a short time when pulling floating matter or pieces. Due to this, a screen is provided at an intake to prevent large floating pieces from being drawn into a suction casing.
However, floating matter or pieces cling to the screen in the water where many other or pieces are floating so that cavitations and the like occur in the water jet propulsion unit, thereby reducing propulsion force. There are some cases where a vessel cannot continue running when the floating matter clinging to the screen increases. In particular, when the screen is of a fixed type navigating the vessel becomes impossible in quite a short time.
To eliminate the floating matter or pieces clinging to the fixed screen, there has been proposed an apparatus in which a movable screen is combined with a fixed screen as disclosed in, for example, Japanese Patent Application Publication No. 54-18475 or Japanese Patent Application Laid-Open No. 6-32288.
Even with a movable screen, once floating matter strongly clings to the screen, it is difficult to remove it and the navigation of the vessel might become impossible.
Furthermore, when floating matter or pieces pass through the screen, are drawn into the suction casing and deposited, the water jet propulsion unit is blocked. To remove the deposited matter or pieces, it is necessary to provide a hand hole in the water jet propulsion unit and to remove the deposited matters manually while the vessel is stopped.